Internal combustion engines may include a fuel system with a fuel rail for distributing fuel to one or more fuel injectors, which may be direct injectors and/or port injectors. In a fuel system operating with direct injectors, a fuel lift pump supplies fuel to a high pressure fuel pump that in turn provides fuel at a high injection pressure to a fuel rail. The fuel rail is coupled to direct injectors that inject the fuel directly into combustion chambers of the engine. In a fuel system operating with port fuel injection, a fuel lift pump supplies fuel at a lower injection pressure to a fuel rail. The fuel rail is coupled to the port injectors, which inject the fuel into the engine intake upstream of intake ports of the combustion chambers. In a port fuel direct injection fuel system, both port injection and direct injection of fuel is performed.
Regardless of the fuel system type, the fuel lift pump can be controlled to output fuel at a substantially constant delivery pressure during what is referred to herein as continuous pump operation or operation in the continuous mode, via application of voltage at a duty cycle of 100% with a voltage level corresponding to the desired constant delivery pressure. When fuel flow demand changes, the voltage level may be adjusted to a different level and held constant or substantially constant at the different voltage level (at a duty cycle of 100%), resulting in a different substantially constant lift pump speed and delivery pressure. In contrast, the fuel lift pump can also be controlled to output intermittent pulses of relatively high pressure in what is referred to herein as pulsed pump operation or operation in the pulsed mode, in which the duty cycle of the voltage applied to the lift pump is less than 100%. During pulsed pump operation, the level of the voltage applied to the lift pump may alternate between a first, higher level and a second, lower level, where the second, lower level is very low (e.g., slightly above 0 V). During application of the first, higher level of voltage to the lift pump, the speed of the lift pump is high and thus the delivery pressure of the lift pump is high, whereas during application of the second, lower level of voltage to the lift pump, pump speed of the lift pump is very low (e.g., at a level slightly above zero, as it may be desirable to maintain supply of voltage to the lift pump rather than intermittently provide zero voltage) and the delivery pressure of the lift pump is very low. As a result, the delivery pressure of the lift pump over time during pulsed mode operation resembles a sawtooth wave, where the duration of time between a trough of the wave and an adjacent peak of the wave following the trough is proportional to a duration of application of voltage at the first, higher level, and where the duration of time between a peak of the wave and an adjacent trough of the wave following the peak is proportional to a duration of application of voltage at the second, lower level.
In contrast to continuous pump operation, pulsed pump operation, in which the fuel lift pump is energized only during the duration of each pulse, is more energy efficient. Further, when pulsed pump operation is performed rather than continuous pump operation, durability of the fuel lift pump may be extended, and maintenance costs of the fuel lift pump may be decreased.
When pulsed pump operation is performed, the controller of the engine may perform either open-loop control or closed-loop control of the pump. When open-loop control is performed, voltage pulses having a predetermined pulse width (and thus, a predetermined duty cycle) may be applied to the lift pump, and measured or inferred pressure downstream of the fuel lift pump (referred to herein as the delivery pressure of the lift pump) does not influence the control. In contrast, when closed-loop control is performed, the delivery pressure is fed back to the controller and influences the duration of subsequent high voltage pulses applied to the lift pump (as well as the duration of the intervals between the high voltage pulses when a voltage slightly above 0 V is applied). In examples where the delivery pressure is measured by a pressure sensor that provides feedback to the controller, degradation of the pressure sensor may shift the reading of the pressure sensor and thereby cause the delivery pressure to deviate from a desired or expected pressure, which may in turn degrade engine operation. As one example, errors within the expected range of sensor output (referred to as in-range errors) are much more difficult to detect than errors outside of the expected range of sensor output (referred to as out-of-range errors). In-range error detection is especially critical when the sensor provides feedback for closed-loop control of pulsed pump operation, as the error will result in incorrect adjustment of the voltage pulses applied to the lift pump.
One approach for addressing fuel pressure sensor in-range error detection is disclosed by Stavnheim et al. in U.S. Pat. No. 6,526,948 B1, which is concerned with diagnosing fuel pressure sensors that are “stuck” in-range. Therein, a controller samples a fuel pressure sensor signal, including pressure peaks and valleys, a number of times. The controller then computes an average pressure value and compares the measured values to the average. If a measured value is within a threshold of the average value, it indicates that the pressure sensor is stuck in-range (that is, not dynamically responding to changes in fuel pressure), and the controller logs an error code. At a certain number of logged errors, the controller initiates a minimum fueling algorithm that supplies just enough fuel to allow the vehicle to be driven out of danger or to a service center.
However, the inventors herein have recognized potential issues with this approach. As one example, the method described above is limited to identifying a degraded pressure sensor that does not respond to pressure fluctuations. However, a degraded pressure sensor may read higher or lower than the actual pressure but still respond to pressure fluctuations. Further, by providing just enough fuel for the vehicle to be driven out of danger or to a service center upon identification of pressure sensor degradation, desired vehicle operation may be unavailable when the pressure sensor is degraded, which may have a negative impact on driver satisfaction.
To address these issues, the inventors herein have identified methods and systems for diagnosing in-range pressure sensor errors and adjusting fuel system operation based on the diagnosis. In one example, the issues described above may be addressed by a method of operating an engine fuel system which comprises, during pulsed mode operation of a lift pump, adjusting a level of voltage applied to the lift pump based on an output signal of a pressure sensor downstream of the lift pump and monitoring the output signal for flattening; and, in response to a detection of flattening, indicating a pressure sensor error and operating the lift pump independent of the output signal of the pressure sensor. In this way, errors occurring within the normal operating range of a pressure sensor arranged downstream of a fuel lift pump can be detected, and fuel lift pump control can be switched from closed-loop to open-loop control upon detection of such errors. While open-loop lift pump control may be less fuel efficient than closed-loop lift pump control, it may not have a substantial impact on drivability.
In order to assure accuracy of the control of the lift pump as well as the in-range pressure sensor error diagnosis, the method may further include dynamically learning a setpoint pressure of a pressure relief valve and a fuel vapor pressure of the fuel system. This may include, during steady state engine operation with a requested delivery pressure of a fuel lift pump below a first threshold, decreasing a duty cycle of voltage pulses applied to a fuel lift pump until flattening of an output signal of a pressure sensor downstream of the lift pump is detected, and storing the pressure at which the output signal flattened as a fuel vapor pressure of the fuel system; during steady state engine operation with a requested delivery pressure of the fuel lift pump above a second threshold, increasing a duty cycle of voltage pulses applied to the lift pump until flattening of the output signal of the pressure sensor is detected, storing the pressure at which the output signal flattened as a setpoint pressure of a pressure relief valve; and adjusting lift pump operation based on the stored setpoint pressure and fuel vapor pressure. Dynamically learning the expected physical maximum and minimum values of the fuel system in this way may improve overall accuracy of the control of the fuel lift pump, and in turn improve the accuracy of pressure sensor error diagnoses.
In yet another example in accordance with the present disclosure, the lift pump may be controlled via a robust closed-loop control strategy. This may include, during pulsed operation of a lift pump, turning the lift pump OFF when a sensed delivery pressure increases to a desired peak pressure or an ON time of the lift pump reaches a calibrated maximum, and turning the lift pump ON when either the sensed delivery pressure decreases to a desired trough pressure or a volume of fuel ingested by the engine reaches a predetermined volume. Such operation may advantageously reduce the possibility of the lift pump becoming “stuck” at a pressure below the setpoint pressure when the lift pump is ON due to the sensor reading low, or at a pressure above the fuel vapor pressure when the lift pump is OFF due to the sensor reading high. Optionally, the robust control strategy may also include calibrating sensor output after detecting that the ON time of the lift pump has reached a calibrated maximum or the volume of fuel ingested by the engine has reached a predetermined volume, so that accurate lift pump control may be performed even when the sensor is degraded.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.